Strain gauge with thermal current control



Nov. 23, 1943. A RUGE 2,334,843

STRAIN GAUGE WITH THERMAL CURRENT CONTROL Origirial Filfid Sept. 16, 1939 \NVENTOR Patent ed Nov. 23, 1943 STRAIN GAUGE WITH THERMAL CURRENT CONTROL 2 Claims.

This invention relates generally to strain gauges and more particularly to a gauge of the type employing a continuous solid filament of electrical conducting material bonded throughout its effective length to the surface of a member under test and whose electrical resistance varies in response to strain, this application being a division of my copending application Serial No. 295,207, filed September 16, 1939.

In a gauge of the foregoing type an electrical current is passed through the wire filament and as a result thermal currents may arise which will distort the accuracy of the gauge, this possibility of distortion being aggravated by the extremely small magnitude of the resistance change in the gauge in response to variations in strain.

It is an object of my invention to provide an improved electrical strain gauge that eliminates or minimizes thermal currents. A further object is to provide an improved electrical strain gauge adapted to eliminate thermal currents in a simple, direct and effective manner, is economical in construction and operation and is compact and rugged.

Other objects and advantages will be more apparent to those skilled in the art from the following description of .the accompanying drawing which is a perspective of my improved gauge applied to a test specimen.

A strain responsive filament I04a of electrical conducting material such as disclosed in my copending application is bonded throughout its effective length to the surface of a member I04d subject to strain. Short pieces I04b of material identical to leads I04 is interposed between the strain filament M411 and leads I04, I04, the pieces I 042) being very small in cross-sectional area relative to their length. The entire filament and lead elements are cemented, as indicatd by the speckled area I05, to a carrying medium or-to a specimen and suitably insulated therefrom if necessary. The material of which filament I04a is made and also its various characteristics are disclosed in my said copending application and hence need not be further described in this application except to point out that the filament is preferably of the approximate order of .001" to .003 in diameter thus showing the extremely minute nature of the electrical element.

If the leads I04, I04 were connected directly to filament I04a at points I040 and should one of these junctures be at a difierent temperature from the juncture for the other lead, then in general a current will pass through the circuit by virtue of thermal currents between the leads and filament arising from a temperature difference at these junctures. It is possible to eliminate thermal current effects from the results of electrical resistance measurements by methods well-known in precise resistance work but such methods are not convenient in strain measurement work. For example, if one terminal lead becomes warmed more than the other, even if the temperature in the specimen is uniform, heat will flow down that lead to warm both the lead and its junction with filament I04a. This may happen without the observer's knowledge thus causing him to make an error in strain reading. However, in my improved arrangement if heat is applied to one of the leads I04 it willhave to flow through pieces I04b before reaching the thermal junction I 040. Since the piece I041) is small in area and relatively long, as compared to the length of filament I04a, substantially all heat applied to lead I04 will flow into the surrounding structure or specimen lo ld before the heat reaches the thermal junction I040. Hence the strain filament I04a will be unafiected by any thermal currents, The thermal dissipating pieces I04b may be made of material diiferent from that of the leads I04 provided it has a zero or negligible thermal-electrical effect against the leads. For example, the leads can be copper and the pieces I04b can be Manganin, while the strain sensitive filament can be any desired material such as Elinvar or Advance. It is not important whether the pieces l04b possess strain sensitivity, the choice here depending upon what overall characteristics are desired.

If the surface of specimen I04d is not at a uniform temperature all over, then it is desirable to have the two junctions I040 close together. I have used straingauges embodying my invention, but without pieces I04b, in circuits which were very difficult to deal with due to thermal currents. After adding pieces I04b the thermal currents dropped to a negligible value even under adverse temperature conditions. For purposes of illustration, Advance wire may be used as the strain filament mm as this has a high thermal electric effect against copper. The pieces I 04b may be copper wire of a length about A and .002" diameter while the leads I04 may be copper about .04" diameter. With such an arrangement the lead wires could be purposely heated far more than they would be normally heated and yet the .002" wire: would add so little resistance that the sensitivity of the usual Wheatstone measuring bridge would not be measurably affected.

From the foregoing disclosure it is seen that I have provided an extremely simple and highly efiective arrangement for eliminating or minimizing thermal currents in a gauge which has a high degree of sensitivity, accuracy and responsiveness without in any way sacrificing these very desirable qualities.

It will of course be understood that various changes in details of construction and arrangement of parts may be made by those skilled in the art without departing from the spirit of the invention as set forth in the appended claims.

I claim:

1. A strain gauge comprising a filament of continuous solid electrical conducting material whose electrical resistance varies in accordance with changes of strain therein, means for bonding said filament throughout its efiective length to a member subject to strain whereby the strain of the filament follows that of said member to efiect a corresponding change of filament resistance, leads for said filament, and other filaments connected to the ends of said bonded filament and to said leads and having a crosssectional area relative to that of said leads for substantially preventing thermal current efiects in said bonded filament.

2. A strain gauge comprising a filament of continuous solid electrical conducting material whose electrical resistance varies in accordance with changes of strain therein,.means for bonding said filament throughout its effective length to a member subject to strain whereby the strain of the filament follows that 01' said member to efiect a corresponding change of filament resistance, leads for said filament, other filaments connected to the ends of said strain filament and to said leads and having a smaller cross-sectional area than said leads thereby to prevent thermal current efifects, and means for bonding said thermal current filaments through their length to the member subject to strain.

ARTHUR C. RUGE. 

